1. Field of the Invention
The present invention relates to an airflow meter provided with a humidity sensor for detecting the humidity of intake air sucked into an internal combustion engine.
2. Description of Related Art
Recently, there is an increasing need to detect the humidity of intake air sucked into an internal combustion engine for, the purpose of increasing the fuel economy and cleaning the exhaust gas of the engine. It is known to mount a humidity sensor on an airflow meter to measure not only the flow rate of intake air but also the humidity of the intake air. For example, refer to Japanese Patent Application Laid-open No. 2010-151795.
However, the humidity of intake air cannot be correctly measured by such an airflow meter if the temperature of an intake duct on which the airflow meter is mounted increases due to heat of an engine, causing the temperature of the humidity sensor to increase excessively, even when the temperature of the intake air is low.
To deal with this problem, the airflow meter described in the above patent document has a structure in which a measurement chamber is provided in the center area of the intake duct so as to be located more distant from the inner wall of the intake duct than the flow rate sensor of the airflow meter is, and the humidity sensor is disposed within the measurement chamber in order to reduce the amount of the heat transmitted from the inner wall of the intake duct to the humidity sensor. However, this structure has a problem in that since the humidity sensor is disposed in the center area of the intake duct, the structural flexibility of the airflow meter is considerably lowered.
In addition, since the flow velocity is high in the center area of the intake duct, the humidity sensor disposed in the center area of the intake duct is subjected to a large amount of dust and water, and as a result, is easily dirtied. To keep the humidity sensor clean, it may occur that the humidity sensor is disposed in the inner passage of the airflow meter. However, in this case, since the heat being received by the intake duct transmits to the member which forms the inner passage, causing the temperature of the inner passage to increase, there is a concern that the temperature of the humidity sensor may change, as a result of which the accuracy of the humidity detection may be lowered.